Cancellation of Home wireless broadband

I am moving abroad in January 2019 and have just tried to cancel my Home Broadband as at the 31st December 2018. I was told that I couldn't as the date was too far away. I then asked that it be cancelled as at 30th November and was told to call back tomorrow. I was insisitent that this was unacceptable, and have now cancelled. My question is why could I not cancel as of 31st December? I will require broadband for the 'gap', and I will be seeking temporary broadband services from your rivals, and when I move back to NZ late 2019, I certainly will not be using your company as my service provider!!